Re: Examples for one-to-one associations?

On Jun 18, 4:49 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> Keith H Duggar wrote:
> > "partial injective function"
>
> > On Jun 8, 9:29 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>Keith H Duggar wrote:
>
> >>>On Jun 7, 2:25 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>>>Gene Wirchenko wrote:
>
> >>>>>On Sat, 5 Jun 2010 20:49:59 -0700 (PDT), hoodwill
> >>>>><chase.saund..._at_gmail.com> wrote:
>
> >>>>>>Examples of one to ones:
>
> >>>>>>1) Spouse
>
> >>>>>    You need a time element.  Even given monogamy, many people have
> >>>>>been married more than once.
>
> >>>>Even more basic than that: Some folks are single.
>
> >>>Why is that an issue? If a person is single then they would not appear
> >>>in the Spouse relation. one-to-one relations can be partial can they
> >>>not?
>
> >>The relative cardinality is 1:zero-or-one not 1:1.
>
> > I don't think that is correct. The cardinality is defined by the
> > domain definitions of the relation, not by the current extent of
> > the relation. Further, since we are not dealing with multivalued
> > theory the cardinality is never of the form "C1:C2 or C1:C3 ..."
> > for a particular domain and relation (where C# are cardinals);
> > rather it is fixed at some C1:C2 regardless of the current
> > extension.
>
> > As far as I know, in mathematics the properties "injective" (ie
> > one-to-one) and "partial" are not mutually exclusive. Please Google
> > "partial injective function" to find both definitions and examples
> > of such functions. For example, the square root function over the
> > natural numbers is considered a partial injective function.
>
> The original question was not about functions (or relations for that
> matter) but about "associations" and "entities".
>
> Most (if not all) ER notations have distinct syntax for 1:1.
> 1:zero_or_1, 1:0_or_more, 1:1_or_more, 0_or_more:0_or_more etc.
>
> 0_or_more is your standard "many".

Oh ok. Don't know how I got sidetracked onto relations ;-)

KHD Received on Sat Jun 19 2010 - 00:25:39 CEST